(1) Field of the Invention
This invention relates to printers, and more particularly it is concerned with a serial printer suitable for use in printing data on a printing sheet by moving a print head in reciprocatory movement along a line on which characters, symbols, etc., are to be printed, so as to perform printing by moving the print head in a normal direction from the beginning of the line or in the reverse direction from the end of the line.
(2) Description of the Prior Art
In this type of serial printer of the prior art, it is the customary practice to move the the print head in reciprocatory movement by a drive unit comprising a step motor as an electric current is passed thereto so as to perform printing both in the normal direction and the reverse direction. The pulse motor rotates at a predetermined speed when a steady-state current of a predetermined voltage is supplied to the drive unit from a battery serving as a power source, to thereby move the print had at a predetermined speed. However, this type of serial printer suffers the following disadvantage. When the voltage supplied to the drive unit drops due probably to a reduction in the electrical charge in the battery, the magnetic force of the stator of the pulse motor decreases and the response of the rotor becomes poor, with the result that the speed of movement of the print head drops below the predetermined level. This causes a variation to occur in the distance covered by the movement of the print head when it performs a printing operation, so that the characters printed by the print head moving in the normal and reverse directions are brought out of alignment with each other.
In view of the disadvantage of the prior art noted hereinabove, it has until now been desired that a serial printer be provided which would be possible, even if the voltage supplied to the drive unit for driving the print head dropped, to avoid a lack of alignment in the positions of the characters, symbols, etc., printed by the print head moving in the normal direction and the reverse direction, thereby enabling the printed characters and symbols to be arranged in an orderly fashion.